This invention relates to conical scanning antennas and, more particularly, to conical scanning antennas of variable scan angle.
Conical scanning antennas find application in numerous radio frequency transmitting and/or receiving system such as air-sea rescue and radar mapping systems. In such systems, it often is either necessary or desirable that the antenna be capable of both wide angle conical scanning and simultaneously be capable of controlling the scan angle over a relatively wide range of cone angles.
Although the prior art has provided arrangements capable of providing the relatively wide scan angles needed in most design situations, each previously proposed arrangement suffers from one or more distinct disadvantage or drawback. For example, the most straightforward approach to achieving wide angle conical scanning consists of moving the entire antenna assembly (including its associated electronics) and, in addition, configuring the transmission lines associated with the antenna and its electronics for movement with the antenna system. Generally, this is accomplished by configuring the transmission lines so that sufficient flexure is possible and/or providing conventional devices such as rotary joints that allow the necessry movement of the antenna assembly. Such an arrangement generally results in a complex system in which the antenna and other portions of the system that must be moved are relatively heavy and, thus, require a relatively large drive system that consumes a substantial amount of electrical energy. Further, flexing of the transmission lines and/or operation of rotary joints can cause other ditrimental effects such as high power arcing, impedance variations and changes in relative phase on adjacent transmission paths.
In another type of prior art system, a "splash plate" is rotated about a predetermined axis and an antenna feed directs electromagnetic energy at the splash plate center of rotation. In arrangements of this type, the conical scan angle depends upon the angle formed between the splash plate and the beam of electromagnetic energy that is supplied by the antenna feed. For example, when the antenna feed is directed along the axis of rotation with the splash plate being positioned so that the angle of incidence between the beam of arriving electromagnetic energy and the splash plate is 45.degree., a 360.degree. scan is produced (e.g., the rotating radiated antenna beam forms a geometric plane, rather than the surface of a cone). When a flat splash plate is utilized, tilting the splash plate about the axis of rotation results in variable conical scanning over a wide range of cone angles. However, high gain operation is not achieved because the antenna gain is substantially equal to that of the antenna feed device (e.g., a feed horn). Although high gain operation can be achieved by utilizing a splash plate that corresponds to an offset parabolic reflector, with the antenna feed being positioned at the focus of the parabolic reflector, such an arrangement generally does not provide satisfactory operation for variable conical scanning applications. Specifically, tilting the parabolic reflector about its axis of rotation causes the feed to be laterally defocused, which, in turn, results in substantial loss of system efficiency (gain) and an increase in spurious directional radiation (increased side lobes), with both effects being proportional to the change in tilt angle.
Another prior art proposal for wide angle concial scanning employs phased antenna arrays in which the antenna elements of a large spherical or hemispherical array are individually fed with electrical signals and the phase relationships between the electrical signals that are fed to the arrayed antenna elements are controlled with respect to time so as to establish the desired conical scan. Such an approach is limited to operation over a relatively narrow band of frequencies and, further, results in a complex and expensive system that is at least as large as the reflectors utilized in other prior art approaches.
In another prior art arrangement, a reflector is defined by a paraboloid of revolution which is symmetrically disposed about a central axis that extends through the vertex of the parabola and the focus of the parabola. The reflector is rotated about a feed axis that is angularly offset from the central axis of the parabolic reflector. In this arrangement, the antenna feed is located at a fixed position on the feed axis and is offset from the focus of the parabolic reflector with the electromagnetic energy that is supplied by the antenna feed being directed at the vertex of the parabolic reflector. As the antenna rotates about the feed axis, both the beam of electromagnetic energy that is reflected from the antenna and the central axis of the reflector move through a conical scanning pattern. Although this arrangement, like the previously mentioned splash plate arrangements, eliminates the need for movement of the antenna feed, the arrangement is limited to conical scanning at relatively low cone angles. Specifically, lateral defocusing of the antenna occurs for wide angle conical can.
Nutating antenna feeds also have been employed in conical scan systems. In a nutating feed antenna, a reflector is utilized that is defined by rotation of a parabola about is focal axis. The antenna feed (e.g., dipole or feed horn) moves in a small circular orbit about the axis of the paraboloidal reflector so that the beam of electromagnetic energy reflected from the reflector is conically scanned. It also is possible to achieve conical scanning by utilizing a circularly polarized antenna feed that is capable of simultaneously operating in both a sum and difference signal mode. The two signal modes are combined in a summing circuit with variable phase offset between the mode signals being employed. Cyclic variation of phase offset causes the antenna feed pattern to rotate thereby resulting in conical scanning of the reflected electromagnetic energy. Although satisfactory in some instances, both of these approaches are limited to small conical scan angles (i.e., small cone angles).